1. Field of the Invention
The field of the invention relates to oscillators and more particularly to voltage controlled oscillators.
2. Related Art
Voltage controlled oscillators (VCOs) are known and are frequently used in radio frequency transceivers for upconverting an information signal from baseband to a particular transmission frequency (i.e., channel) or for downconverting an information signal from a particular transmission channel to baseband.
VCOs typically operate based upon the use of tank circuits and upon the influence of a variable capacitance provided by a varactor. As is well known, tank circuits are devices that resonate at frequencies determined by inductive and capacitive elements. Changing the values of the inductive or capacitive elements changes the resonant frequency of the tank circuit, thereby changing the output frequency of a VCO relying upon the tank circuit.
The varactor of a VCO may be a diode or some other similar device. As is known, the capacitance across some diodes changes as a function of a reverse-bias voltage. Where the reverse-biased diode is connected to an inductor, the resonant frequency of the tank circuit can be independently controlled by the voltage across the diode.
While VCOs are effective, low voltage VCO designs, relying upon on-chip varactor diodes, typically suffer from performance impediments such as frequency centering errors and large variations in tuning constants. Frequency centering errors can be reduced by providing a large tuning sensitivity range, but this can contribute to phase noise. Because of the importance of VCOs in communications, a need exists for a better method of improving the tuning linearity and centering frequency error of VCOs.
This invention provides for controlling an output frequency of a voltage controlled oscillator relative to a reference frequency. The method includes the steps of detecting a phase error between a divided output of the voltage controlled oscillator and the reference frequency, pumping a frequency control input of the voltage controlled oscillator with the phase error and adjusting a pumping gain based upon a magnitude of the frequency control input to the voltage controlled oscillator.
The apparatus includes a phase detector adapted to detect the phase error between the divided output of the voltage controlled oscillator and the reference frequency and a charge pump adapted to pump the frequency control input of the voltage controlled oscillator with a phase error. The apparatus also includes a gain controller adapted to adjust the pumping gain based upon the magnitude of the frequency control input to the voltage controlled oscillator.
Other systems, methods, features and advantages of the invention will be or will become apparent to one of skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.